Coordinate measuring apparatus are utilized more and more in the immediate manufacturing area and are, for example, connected with the machine tools of a flexible manufacturing system.
However, it is not possible to carry out length measurements at a reference temperature of 20.degree. C. in the manufacturing area. Measurements at the reference temperature can be carried out only in specially climatized measuring rooms. For this reason, the suggestion has already been made that the entire coordinate measuring apparatus in the manufacturing area be encapsulated in a climatized cabin. However, this requires a considerable effort and the cabin hinders the incoming supply of the workpieces to be measured.
The probing devices must be repeatedly calibrated in relatively short time intervals when measurements are made outside of the climatized rooms or cabins. Even a slight deviation of the reference temperature at which the calibration was once carried out leads to intolerable measurement errors because of the thermal linear expansion of the sensing devices. An aluminum probing device having a length of 100 mm expands, for example, by 2.5 .mu.m for a temperature increase of 1.1.degree. C. This error can already exceed the permissible measurement uncertainties of the coordinate measuring apparatus used.
As a rule, for a measuring operation for a complete workpiece, several different sensing devices are exchanged sequentially and these probing devices are taken either from a storage closet or from a magazine built up at the edge of the measuring area. For this reason, the temperature of the probing devices can change between measurements. Accordingly, a new calibration must be made after each probing device exchange in order to obtain accurate measurements. This slows the entire measuring operation and is especially disadvantageous in a manufacturing area where rapid measuring results are required.
The above-mentioned difficulties can be avoided by making the probing device from a material having a low expansion coefficient such as Invar. However, sensing heads and probes made of Invar are more complex to manufacture. A conversion of the probing device to this material is therefore not possible in a simple manner.